The present invention relates to a method and an apparatus for manufacturing foams using carbon dioxide dissolved under pressure as blowing agent, wherein the material to be foamed is mixed under pressure with carbon dioxide (preferably with liquid carbon dioxide) and is then expanded with foam formation. There foamable materials are used as liquid starting products for plastics materials, which cure to the foamed plastic by virtue of a polyaddition or a polycondensation reaction commencing after the foaming. The invention relates in particular to polyurethane foams.
For the manufacture of polyurethane foams at least one of the reactive components (i.e., compounds containing polyisocyanate and isocyanate-reactive hydrogen atoms, in particular polyols) is blended with a liquid or gaseous blowing agent and is thereafter mixed with the other component. The mixture obtained is fed either discontinuously into a mold or continuously onto a conveyor belt, where the mixture foams up and cures.
A number of methods have found wide application in the industry for the production of the foam. For example, liquids (such as low-molecular weight chlorofluorocarbons, methylene chloride, pentane and the like) which evaporate out of the still liquid reactive mixture and form bubbles (physical foam production) are often used. Alternatively, air can be injected into the reactive mixture or into one of the components (mechanical foam production). Finally water can be added as a blowing agent to the polyol component in the case of polyurethane foams, which after mixing with the isocyanate component releases carbon dioxide as foam gas by reaction with the isocyanate (chemical foam production).
For reasons of environmental compatibility and occupational health and because of the comparatively high solubility of liquid carbon dioxide in the polyol component, liquid carbon dioxide has already been proposed as blowing agent many times (see, e.g., British patent 803,771; U.S. Pat. Nos. 3,184,419 and 5,120,770; and published European patent application 645,226). Carbon dioxide has however not found acceptance to date in the industry, due apparently to the difficulties of producing regular foams during the required expansion of the reactive mixture to pressures of between 10 and 20 bar. Directly after the expansion, the carbon dioxide evaporates relatively suddenly, so that a violent increase in volume of the reaction mixture by a factor of, for example, about 10, takes place, which is difficult to control. In addition, the reactive mixture is susceptible to delays in the release of the carbon dioxide (which can lie 3 to 6 bar below the equilibrium vapor pressure of CO.sub.2 at the respective temperature) so that sudden explosive-type carbon dioxide releases occur, resulting in large bubbles or voids being incorporated in the foam.
According to German patent 2,628,785, it has been proposed, in order to produce nuclei for the carbon dioxide release, to introduce air into the polyol component before carbon dioxide is dissolved in the polyol.
According to European patent 145,250, the capacity of the carbon dioxide to form adducts with water and other low-molecular liquids is used to achieve a delayed release of the carbon dioxide from the reactive mixture, so that the start of the foam formation through release of the carbon dioxide is delayed until after the pressure relief of the reactive mixture. After decomposition of the adduct, the water is available as an additional chemical blowing agent. However, the controllability of the foam formation in a large-scale process is not significantly improved in this way, since both adduct formation and adduct decomposition are extremely unstable compared with the other conditions prevailing in the reactive mixture, unless the adducts manufactured in a separate step were to be used in combination with low-molecular tertiary amines, which apparently have an exceedingly long decomposition time compared with spontaneously formed water CO.sub.2 adducts.
Combinations of physically dissolved carbon dioxide and other low-boiling physical or chemical blowing agents such as water or chlorofluorocarbons are already also proposed according to European patent 89,796.
None of these proposals has led to industrially applicable solutions for the use of CO.sub.2 dissolved physically under pressure as a blowing agent for polyurethane foam manufacture.
The investigations on which the present invention is based start from the idea that the conditions under which the polyurethane reactive mixture containing pressurized CO.sub.2 is expanded have a significant influence on the foam formation.
According to U.S. Pat. No. 3,184,419, the reactive mixture containing pressurized carbon dioxide exiting from the mixing device is apparently expanded suddenly by means of a valve. According to European patent 145,250, the expansion should take place gradually, wherein the gradual reduction in pressure can take place during the flow of the reactive mixture containing carbon dioxide through a tube. The fact that the release of part of the gas still takes place in the tube is not necessarily regarded as a disadvantage, since bubble nucleation can be supported in this way. It has been found in the course of the investigations on which the present invention is based, however, that such a premature, i.e. spontaneous and non-induced, bubble nucleation is somewhat disadvantageous in terms of the pore structure of the foam, since a foam is generally obtained from such a spontaneous bubble nucleation which not only exhibits a widely varying pore structure, but also sizeable bubbles and voids.
The concept of delayed carbon dioxide release, i.e. release only after placing of the reactive mixture on the molding means (conveyor belt or mold) by forming an adduct of the carbon dioxide with compounds containing hydroxyl groups, also does not lead to a controlled foam formation, because the adducts are extremely unstable under foaming conditions. Consequently considerable amounts of non-adducted carbon dioxide are also always present, which are released spontaneously ahead of time with the corresponding presence of bubble nuclei.
On the basis of these and other observations, we have determined the following requirements for the development of a method for manufacturing foams from two-component reactive plastics using carbon dioxide dissolved physically under pressure as blowing agent:
1. The expansion of the reactive mixture containing carbon dioxide in dissolved form (namely from a pressure at which the mixture is subsaturated up to a pressure at which the mixture is supersaturated with carbon dioxide) must take place within times sufficiently short that release of the carbon dioxide has not occurred before the components are mixed. PA1 2. The formation of bubble nuclei in the liquid two-component reactive mixture must be controlled in such a way that each bubble nucleus produced can participate in identical manner in the release of the carbon dioxide from the reactive mixture, so that as regular pores as possible are produced. PA1 3. Already during the induced bubble nucleation, bubble nuclei must be produced in sufficient numbers corresponding to the pore count in the cured plastics foam. PA1 4. The nucleation must be brought about with a maximum possible supersaturation of the reactive mixture with dissolved carbon dioxide, i.e. directly after the expansion to approximately ambient pressure.